Gamma thermometer axial apparatus and method for monitoring reactor core in nuclear power plant

ABSTRACT

A method for collecting data regarding the operating condition of a nuclear reactor core including: positioning a first linear array of gamma thermometer (GT) sensors in a first instrument housing, wherein the GT sensors are arranged asymmetrically along the first linear array; positioning a second linear array of GT sensors in a second instrument housing, wherein the GT sensors are arranged asymmetrically along the second linear array and wherein the second linear array of GT sensors is asymmetrical with respect to the first linear array of GT sensors; positioning the first instrument housing in the reactor core at a first core location and positioning the second instrument housing at a second core location symmetrical with respect to the first core location; collecting core condition data from at least one of the GT sensors in the first linear array of GT sensors, and applying the collected core condition data as data collected from the second linear array. Detector calibration can be accomplished by using the adjacent GT sensor, the closest GT sensor in the string (if the adjacent GT sensor is unavailable) or by using the GT sensor in a symmetric string that has a close axial height to the adjacent GT sensor.

BACKGROUND OF THE INVENTION

This invention relates to monitoring the reactor core in a nuclear powerplant and, particularly, to arranging gamma thermometers in the core ofa boiling water nuclear reactors (BWR).

A typical BWR nuclear power plant includes nuclear instruments thatmonitor the condition of the reactor core. The signals generated bythese instruments are used to maintain the reactor core within allowableoperating conditions. The instrument signals may be processed by a coremonitor software that determines the 3-Dimensional (3D) nodal powers andthe 2-Dimensional (2D) bundle flows. The 3D nodal powers and 2D bundleflows may then be used to determine thermal margins within the reactorcore. The operators may use the determined thermal margins to makeadjustments to the core operating conditions so not to exceed theallowable operating conditions. Further, the 3D nodal powers and 2Dbundle flows may be used by plant operators to confirm that the reactorcore is operating within allowable operating conditions.

The instruments for a typical BWR nuclear reactor include a TransverseIn-core Probe (TIP) system and a Local Power Range Monitor (LPRM) and/orStart-Up Range Neutron Monitor (SRNM) systems. The available types ofTIP instruments comprise instruments to measure neutron thermal flux andinstruments to measure gamma flux. LPRM instruments generally measureneutron thermal flux.

TIP and LPRM instruments are arranged in a core to take axialmeasurements at fixed radial locations in the core. Conventionally, theTIPs are mechanically moved in and out of the core to calibrate theLPRMs and, particularly, to calibrate individual detectors in each LPRM.During the calibration process, a TIP is positioned next to a detectorof a LPRM and the LPRM gain electronics are adjusted to cause the LPRMdetector to generate an output signal equivalent to an output signalfrom the adjacent TIP. In addition, the TIPs may provide processedoutput signals indicating the neutron thermal flux and gamma flux atvarious elevations in the reactor core, such as at elevations at sixinch (15 centimeters) intervals. The flux measurements taken at variouselevations of the core provide axial information regarding the powershape in the core at non-LPRM core locations.

Maintaining and operating the TIP mechanical system to raise and lowerthe TIPs is expensive. Gamma Thermometers (GT) sensors have been usedinstead of TIPs. Unlike the TIPs that were moved in and out of the core,the GT sensors are positioned at fixed axial locations in the core.Similar to TIPs, the GT sensors are used to calibrate the LPRMs. Becausethe GT sensors are at fixed axial locations, the expense of a mechanicalmovement system to raise and lower the TIPs has been eliminated for thestationary GT sensors.

In a conventional application, seven or more GT sensors are arranged asa linear array, such as on a vertical string. These vertical arrays ofseven GT sensors are positioned at various fixed elevations in thereactor core. The fixed elevations for the GT sensors are manufacturingdetermined and correspond to fixed axial positions on the stringsupporting the GT sensors.

Fabricating the GT sensors on the strings is problematic due to thenarrow tolerances for axial placement of the GT sensors in each string.Each GT of a string must be positioned precisely on the string to bepositioned in the core at the elevations to which they are assigned. TheGT sensors in each vertical string are each positioned within narrowvertical tolerances to assure that each GT sensor is positioned at itsassigned axial position, e.g., adjacent a LPRM when the GT sensor isplaced in the core. The narrow vertical tolerances for the GT stringsare necessary so that the LPRMs can be accurately calibrated. Eachstring of GT sensors is permanently fixed in the core after the array isproperly positioned and vertically aligned with the LPRMs.

To expand the vertical tolerances for the GT strings would, incombination with the inherent uncertainty in any nuclear measurementsystems, e.g., the LPRMs, create uncertainties in the determination ofthe operating conditions of a reactor core. An increase in theuncertainties in the determination of core operating conditions willlikely lead to a narrowing of the reactor core operating limits as theoperating margins are increased to compensate for the increaseduncertainties. The increase margins can result in additional reactorfuel cost as the acceptable operating conditions are narrowed.

The narrow axial tolerances applied to the strings of GT sensors areproblematic with respect to the manufacturing of these arrays. Thetolerances reduce the number of GT sensors that can be accuratelypositioned on a GT string to, for example, seven GT sensors. The limitednumber of GT sensors that can be manufactured on each string reduces theamount of core information that can be sensed by the GT sensors.

The amount of information regarding the axial power shape of a core thatcan be sensed by a GT string is dependent on the number of GT sensorsvertically arranged on the string. Each GT sensor collects data at aparticular axial position on the string, which corresponds to anelevation in the core. Limiting the number of GT sensors on each GTstring limits the core elevations for which there is data from GTsensors.

The amount of information regarding the axial power shape increases asthe number of GT sensors on a string increases. For example, seven GTsensors on a string provides less information regarding the axial powershape at various core elevations than would twenty GT sensors on astring. Increasing the information that is sensed by the GT sensorsregarding the axial power shape in a core reduces the uncertainty ofthat power shape. A reduction in the uncertainty of the power shapeallows for a corresponding reduction in the margins applied to the coreoperation limits. Reducing the uncertainty margins, allows for the coreto be operated at conditions that are safe and more efficient respect tofuel consumption.

It is conventional for the axial locations of each GT/sensor in a stringto be specified prior to the manufacture of the string. In addition,each GT string for a core is manufactured such that the GT sensors arearranged at the same axial locations on each string. Thus, GT sensorsare arranged at the same core elevations for every axial location of theGT arrays. Because these GT elevation locations are fixed, the GTelevations are typically hard coded into the core monitoring software.The core monitoring software does not allow for GT sensors to bearranged at core elevations outside the assign axial positions for eachsensor and the narrow tolerances predefined for the GT sensors.

There is a long felt need for an arrangement of GT sensors that can bereadily manufactured and provide an increased amount of informationregarding the axial power shape of a core.

BRIEF DESCRIPTION OF THE INVENTION

A method has been developed for collecting data regarding the operatingcondition of a nuclear reactor core including: positioning a firstlinear array of gamma thermometer (GT) sensors in a first instrumenthousing, wherein the GT sensors are arranged asymmetrically along thefirst linear array; positioning a second linear array of GT sensors in asecond instrument housing, wherein the GT sensors are arrangedasymmetrically along the second linear array and wherein the secondlinear array of GT sensors is asymmetrical with respect to the firstlinear array of GT sensors; positioning the first instrument housing inthe reactor core at a first core location and positioning the secondinstrument housing at a second core location symmetrical with respect tothe first core location; collecting core condition data from at leastone of the GT sensors in the first linear array of GT sensors, andapplying the collected core condition data as data collected from thesecond linear array.

A pair of linear arrays of gamma thermometer (GT) sensors have beendeveloped that are arranged in a nuclear reactor core, the paircomprising: a first linear array of GT sensors, wherein the GT sensorsare arranged asymmetrically along a length of the first linear array; asecond linear array of GT sensors, wherein the GT sensors are arrangedasymmetrically along the second linear array and wherein the secondlinear array of GT sensors is asymmetrical with respect to the firstlinear array of GT sensors; the first linear array positioned in thereactor core at a first core location and the second instrument housingpositioned at a second core location symmetrical with respect to thefirst core location.

A method has been developed to collect and present data from gammathermometer (GT) sensors indicative of a nuclear reactor core, themethod comprising: forming a plurality linear GT arrays of GT sensors,wherein an axial positions of the GT sensors in each array is notpredetermined prior to forming the array; determining the axial positionof each of a plurality of GT sensors arranged in each of the linear GTarrays; storing the axial positions for each GT sensors in a data fileassociated with the linear GT array; loading the data file for each ofthe linear GT arrays into a core monitor software positioning; for eachof the linear GT arrays, the core monitor software determines theelevation in the core of each of the GT sensors based on the axialpositions in the data file; positioning the linear GT arrays in thecore; collect data regarding an operating condition of the core from theGT sensors for each linear GT array, and the core monitoring softwareusing the collected data to generate a presentation of a core conditionat various core elevations. The method may further include: positioningeach of the linear GT arrays in a separate instrument tube; positioningdetectors for a Local Power Range Monitor (LPRM) in each of theinstrument tubes; for each instrument tube, identifying one of the GTsensors of the array adjacent to each of the detectors, and calibratingeach of the detectors by either using the identified adjacent GT sensoror a non-adjacent GT sensor that is closest to the detector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a reactor core shown in a top downview.

FIG. 2 is a perspective view of an instrument tube arranged between fuelbundles, and showing the tube in a cut-away view to expose sensors inthe tube.

FIG. 3 is a schematic diagram of an instrument tube housing sensors foran LPRM and a GT string with a relatively large number of GT sensors.

FIG. 4 is a flow chart of a fabrication method and registration methodfor the GT strings.

FIG. 5 shows an exemplary three dimensional (3D) map of a power mapgenerated by data collected from the GT sensors.

FIG. 6 is a schematic diagram of a pair of instrument tubes atsymmetrical positions in the core and having GT strings with GT sensorsarranged asymmetrically on the string.

FIG. 7 is a chart showing an example of expected instrumentation signals(such as from LPRM, GT sensors) at various axial elevations anddemonstrates the similarities of two linear arrays of instruments atsymmetric core locations.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic illustration of a reactor core 10 contained in areactor vessel 12 of a BWR. The core 10 comprises, for example, hundredsof fuel bundles 14 (identified in FIG. 1 by white squares with blackborders). Each fuel bundle 14 may include an array of fuel rods, waterrods and channels. Control rod blades (not shown) are inserting betweengroups of bundles, e.g., between 2×2 arrays of bundles. Water in thereactor vessel 12 surrounds the rods. Heat generated by nuclearreactions is transferred from the rods to the water circulating throughthe core, boiling some of the water. The heat generated in the core iscarefully controlled to maintain safe and efficient reactor operationsin the core.

The vessel 12 is typically a cylindrical water filled chamber sealedwithin the BWR. The fuel bundles are submerged in the water in thevessel 12, wherein the vessel may have a water filled depth ofapproximately 12 feet (4 meters) or more. FIG. 1 is a top down view of avessel that shows the two-dimensional (2D) array of hundreds of fuelbundles 14 arranged in the core. The 2D array of fuel bundles may bearranged in rows and columns and have a generally circular perimeter.The center 16 of the array of fuel bundles is referred to as the radialcenter of the array. Fuel bundles 14 near the center 16 of the arraytend to experience more severe operating conditions than do fuel bundlesnear the outer periphery 18 of the array. Fuel bundles at a common radii20 in the array will typically experience similar core operatingconditions.

Various instruments are positioned in the core vessel to monitorconditions in the core. These instruments may include flux measuringdevices that measure neutron and gamma flux in the core and provide dataindicative of the power of the reactor. For example, startup rangeneutron monitor (SRNM) channels 22 (black squares in FIG. 1) and localpower range monitor (LPRM) systems 28 (black circles). The SRNM channelsmeasure low power ranges of neutron flux at their respective channellocation in the core. The core may have other power monitors, such aslocal power range monitor (LPRM) channels that measure neutron and gammaflux in the core. In addition, TIP or GT instruments may be arrangedside-by-side with the LPRMs and SRNMs.

The instruments 22 and 28 are arranged at predetermined positions in thecore, such at predetermined radial positions with respect to the center16 of the core. The instruments may be arranged symmetrically in thecore such, as is shown in FIG. 1. Each type of instrument, e.g., LPRMand SRNM, may be arranged symmetrically in the core. The monitoringinstruments, e.g., LPRM, SRNM, TIPs and GT sensors, measure neutronand/or gamma flux at various radial positions and elevations in thecore.

The core has a line of symmetry 31 extending through the center 16 ofthe core 10. The core is symmetrical about the line of symmetry. Theline of symmetry is typically a line extending at 45 degrees from the1:30 position to the 7:30 position on the core, as shown in FIG. 1. Inparticular, the conditions in the arrangement and condition of the coremirrored on opposite sides of the line of symmetry. With respect to theinstruments 33 at the same (common) distance from the line of symmetryand along a line perpendicular to the line of symmetry receive a uniformamount of flux from the core, provided that they are at the sameelevation in the core. By way of example, instruments 33, e.g., a pairof GT string, are the same distance from the line of symmetry 31 along aline perpendicular to the line of symmetry. GT sensors in this pair ofGT strings at the same elevation in the core will receive the sameamount of flux from the core.

FIG. 2 shows a conventional arrangement of fuel bundles 14, surroundinga flux measuring instrument 30, and adjacent a control rod blade 32. Theflux measuring instrument 30 may be housed in a sealed hollow instrumenttube 34 containing a linear GT string array 36 of individual sensors,including Gamma Thermometers (GT sensors) 38 and neutron flux detectors40 for a LPRM or SRNM.

The instruments 30 are typically arranged between a group of fouradjacent fuel bundles. Each instrument 30 extends substantially thelength of the fuel bundles, preferably at least the length of the activeportion of the fuel rods in the bundle. The instruments are at the sameelevation in the core as the fuel bundles and are preferably fixed inthe core during core nuclear operation.

The instruments 30 are arranged in a core to collect data regardingneutron and gamma flux levels at various radial positions in the core.At each radial position in the core, the instruments collect flux leveldata at various elevations in the core. Within each instrument tube 34,individual measuring sensors 38, 40 are arranged at various axiallocations along the length of the tube. When the tube is in the core,these axial locations in the sensors correspond to various elevations inthe core. The sensors provide data on flux levels for each of the axiallocations of the sensors in the instrument tube 34 and at the coreelevations corresponding to the axial locations.

In each instrument tube 34, the sensors may include, for example, GTsensors, LPRMs and SRNMs. The sensors may also include TIPs, that arearranged adjacent the instrument tube. However, TIPs are typically notfixed in the core, and are moved mechanically in and out of the core bythe operators of the plant. TIPs are preferably not used because theyrequire complex and expensive mechanical conveyors to move the TIPswithin the core. A benefit of the GT sensors over the TIPs is that GTsensors may be fixed in a tube housing 34 with the LPRMs or SRNMs.

Each GT sensor 38 generates output signals providing data of gamma fluxlevels at the radial location on the string for the sensor whichcorresponds to an elevation in the core. The GT sensors in each tubehousing 34 may be used to calibrate the LPRMs 40 in the same tube. Tocalibrate the LPRMs, the string 36 of GT sensors is positioned in thetube such that a GT sensor is axially aligned next to a LPRM in thetube. To calibrate a LPRM, the output signal of the LPRM is adjusted tomatch the output signal of the GT sensor adjacent to the LPRM. Theoutput signal of the LPRM may be adjusted by changing the gain of theelectronics in the LPRM that processes the signal from the LPRM. Inaddition, the GT sensors may be used to acquire flux data from the core.

The arrangement of GT sensors 38 in the tube 34 shown in FIG. 2 arearranged at predetermined locations along the GT string 36. The GTstring and its GT sensors are conventional. These predeterminedlocations are specified prior to the manufacturing process for the GTstring. The string is manufactured such that the GT sensors 38 arepositioned at the predetermined axial locations along the string.Typically, a relatively small number of GT sensors, e.g., seven, arepositioned on a string at specified axial locations corresponding to thelocations of the sensors for the LPRM. The predetermined axial stringpositions for the GT sensors are subject to narrow tolerances to ensurethat the GT sensors are positioned adjacent the LPRM sensors.Positioning the GT sensors at the same axial position as the LPRMsensors is needed to accurately calibrate each LPRM sensor with anadjacent GT sensor. The greater the axial distance between the GT sensorand LPRM sensor, the greater the uncertainty in the calibration of theLPRM sensor.

Due to restrictions of the manufacturing process for strings of GT andthe tight axial tolerances for positioning GT sensors on the string,conventional strings about seven GT sensors is the maximum number ofsensors that can be economically manufactured using conventional stringmanufacturing processes. In addition to calibrating the LPRM sensors,flux data from the GT sensors may be used to determine the power shapeof the core. The limited number of GT sensors reduces the flux data thatcan be acquired at different core elevations by the GT sensors. Theresolution of the determined power shape of the core is dependent, inpart, on the number of GT sensors on each string. With only seven GTsensors on each string, the resolution of the power shape (as determinedby the GT sensors) in an axial direction of the core, e.g., at differentcore elevations, is relatively coarse.

The resolution of the determined power shape of the core may beincreased by increasing the number of GT sensors on each GT string. Anew GT string structure and method for fabricating the strings has beendeveloped that eliminates the prefabrication axial positioningrestriction on GT sensors in a string. By eliminating this positioningrestriction, a relatively large number, e.g., sixteen or greater, of GTsensors may be arranged in a string.

FIG. 3 is a schematic diagram of an instrument tube housing sensors 34for an LPRM and a GT string 42 with a relatively large number of GTsensors 38. The tube housing, LPRM and GT sensors may be conventional.The fabrication process for the GT string may also be conventional, withthe exception that the fabrication process is not limited to placing theGT sensors at predetermined axial positions on the string. Thefabrication process may include positioning a relatively large number ofGT sensors, e.g., sixteen or more, in a metallic rod of, for example,having a diameter of two inches (5 cm) and length of one foot (30 cm).

FIG. 4 is a flow chart of a fabrication method and registration methodfor the GT strings. In step 60, the number of GT sensors placed in therod may be as many as can fit in the pre-extruded rod. In step 62, therod with the GT sensors is stretched through a conventional extrusionprocess to a length of, for example, 15 feet (4.5 m) and a diameter of0.25 inches (6 mm). During the extrusion process, the GT sensors aresomewhat arbitrarily dispersed along the axial length of the rod, as isshown in the stretched rod 42 shown in FIG. 3.

After the extrusion process, the axial position 44 of each GT sensor 38in the string is measured using conventional GT sensor detectioninstruments and methods, in step 64. The axial position 44 of each GTsensor is determined to a high degree of accuracy, e.g., to within 1 to5 mm. The axial position of each GT sensor is registered and stored forfuture reference when using the particular GT string, in step 66. Theaxial position of each GT sensor is stored in a computer data file thatis associated with the corresponding GT string.

The data file of the axial positions of each GT sensor in a GT string isloaded into a computer system for the reactor core that includes a coremonitoring software program, in step 68. This program may be aconventional system for monitoring reactor core operations, analyzingdata from instruments and sensors monitoring the core and generatingmaps showing the power shape of the core in three dimensions (3D),including in radial directions and elevations in the core. Conventionalcore monitoring programs have hard coded axial positions as to known GTsensor positions in each GT string. For present purposes, the program ismodified to accept an input file of the GT sensor positions for each ofthe GT strings in instrument tubes arranged in the core. In particular,the software program reads the data file of the axial positions of GTsensor for each GT string and determines the elevation in the core foreach GT sensor, in step 70. The software associates the elevation ofeach GT sensor with the radial position of the GT string in the core(and optionally the angle of the radii associated with the sensor).Knowing the elevation and radial position of each GT sensor, the coremonitoring software uses the data collected by the sensors to generate apower shape map of the core, in step 72.

An exemplary 3D power shape graph 56 is shown in FIG. 5. The 3D powershape graph presents radial power distributions at different axialelevations (E) in the core corresponding to the elevation of the GTsensors on the GT strings. Core locations 57 may be color coded torepresent the core power at each location. For example, a core location57 shown in red may indicate a higher power level than another corelocation shown in blue. The core locations are shown in the 3D graph 56at various core elevations and at different core locations on eachelevation.

In addition, the manufacturer of the instrument tube 34 or the coremonitoring software identifies the GT sensor 46 in a string adjacenteach detector of the LPRM 40 in the tube, in step 74. The identified GTsensor adjacent each detector is used to calibrate the detector, in step76. Because of the relatively large number of GT sensors arranged in thestring, there will be a GT sensor adjacent each LPRM detector. The GTsensors adjacent each LPRM detector can be determined knowing the axiallocation of both the GT sensors and the LPRM detectors.

If the adjacent GT sensor fails or is unavailable, the closest GT sensorto the detector will be used to provide the detector calibration. Usinga GT sensor that is further away from the detector, increases theuncertainty of the calibration.

Having the manufacture of the GT strings determine the as-built axiallocations of the GT sensors after the string is fabricated allows forthe removal of the requirement that a particular GT sensor be positionedat a specific axial location in the string. Without this requirement,many more GT sensors, e.g., two to three times more GT sensors, can beplaced in a GT string than when the requirement is imposed. Byincreasing the number of GT sensors in a string increases the elevationsin the core at which GT sensors collect flux data regarding the power ofthe core.

FIG. 6 is a schematic diagram of a pair of instrument tubes 52, 54 atsymmetrical positions (see instruments 33 in FIG. 1) in the core andhaving GT strings 56, 58 with GT sensors arranged asymmetrically on thestring. The pair of instrument tubes 52, 54 that are arranged at acommon distance from the line of symmetry 31 (FIG. 1). These tubes 52,54 both have a GT string 56, 58 of GT sensors and a LPRM 40. The GTstrings 56, 58 detect the same core conditions, e.g., gamma flux,because they are at symmetric positions in the core. Symmetric positionsin the core include positions at the same core elevation, and samedistance from the line of symmetry along a line core perpendicular tothe line of symmetry. For symmetrically situated GT strings, it can besafely assumed that the flux data collected by the GT sensors on thestrings should be the same, except for elevation variations of the GTsensors.

The asymmetrical arrangement of GT sensors in the strings may beintentional and based on axial locations of GT sensors determined beforethe fabrication of the GT string. Because of the predetermined axiallocations of the GT sensors, the number of sensors that can be arrangedin the string is limited to, for example seven GT sensors. The GT string56 on the left side of FIG. 6 has more GT sensors towards the top halfof the string than in the bottom half. Similarly, the GT string on theright side of FIG. 6 has more GT sensors in the bottom half of thestring than in the top half. Another form of asymmetric locations is tohave the same number of GT sensors on two strings at symmetric and theaxial locations of GT sensors on each string be different.

The asymmetrical axial arrangement of GT sensors in each string resultsin at least some of the GT sensors in one string 56 being at elevationsin the core at which there is no corresponding GT sensor in the otherstring 58. At those core elevations, only one GT string is acquiringdata regarding the flux in the core. In addition, there are fewer GTsensors than detectors 40 for the LPRM. Calibrating the detectorswithout an associated GT sensor is problematic. The LPRM detector axialpositions are identical for both instrument tubes 56, 58. Thesymmetrically positioned GT strings 56, 58, have GT sensors arrangedsuch that collectively for both strings there is a GT sensor at each ofthe LPRM detector axial positions in the instrument tube. The coremonitor software identifies GT sensors that are directly adjacent a LPRMdetector, e.g., at the same elevation, or the GT sensors closest to theLPRM detector. If only one GT sensor is identified (for example a GTsensor is at the same elevation ad the detector), the detector iscalibrated by adjusting the electronics associated with the LPRMdetector until the detector outputs the same signal level as beingsensed by the identified GT sensor. If multiple GT sensors areidentified as being proximate a LPRM detector, the core monitor softwaremay interpolate the GT sensor signals, such as by a weighted averagebased on the axial distance from each GT sensor to the same elevation ofthe detector. The weighted average of GT sensors is used by the softwareas a reference signal to which is matched the output of the LPRMdetector by adjusting the electronics for the LPRM detector.

The core monitor software infers that GT strings at symmetric locations,e.g., at the same distance from the radial symmetry axis, experience thesame core conditions, including core flux. Based on this inference, thecore monitor software applies the GT sensor data from one string (the“originating string”) as data that is collected a both the originatingstring and data collected at another string at a symmetrical corelocation with the originating string, unless the other string has a GTsensor at the same core location. Similarly, a LPRM detector 40 thatdoes not have a directly adjacent GT sensor may be calibrated using thedata from the GT sensor at the same or close elevation as the detectorand in an instrument tube at a symmetrical core location.

FIG. 7 is a chart showing the strong correlation of data from GT sensors(GT-1, GT-2) and data from detectors (LPRMs-string-1, LPRM-string-2),where the sensors and detectors are at almost the same axial locationsin an instrument tube, and the tubes are symmetrically arranged in thecore. In particular, a first instrument tube with GT sensors (GT-1 anddetectors (LPRMs-string-1) is symmetrically arranged in the core with asecond tube with GT sensors (GT-2) and detectors (LPRMs-string-2). TheGT sensors at the same axial height in both tubes have substantially thesame expected data signal output. Similarly, the detectors (LPRMs) inboth tubes and at the same axial height have the same expected detectorsignal output, which is equal to the GT sensor signals for thecorresponding elevation. Because of the similarity of signals, a GTsensor in one tube can be used to calibrate a detector in another tube,provided the detector is at the same elevation as the GT sensor and thetube with the detector is symmetrically arranged in the core with thetube having the GT sensor.

The core monitor software identifies GT strings in the core atsymmetrical core locations, in step 78 of FIG. 4. For symmetrical GTstrings, the controller identifies core elevations at which one but notboth of the symmetrical strings have a GT sensor, in step 80. For theone GT sensor at each of the identified core elevations, apply thesensor data as originating from both symmetrical strings in analyzingthe condition of the core, such as in generating a power shape of thecore. To calibrate the LPRM detectors, the core monitor softwareidentifies detectors 40 that do not have an adjacent GT sensor in thesame instrument tube, in step 84. To calibrate each of the identifieddetectors 40, use the data from the GT sensor at the same axial positionas the detector but from an instrument tube symmetrically positioned inthe core.

The two schemes disclosed herein for increasing the effective number ofGT sensors monitoring a core reduce the uncertainty in the nuclearmeasurement system relative to the current economical optimum of aboutonly seven GT sensors per GT string. The first scheme increases thenumber of GT sensors in each string, but has a risk of not positioning aGT sensor directly adjacent a LPRM detector for calibration purposes.The second scheme uses a reduced number (albeit a conventional number)of GT sensors but relies on an asymmetrical arrangement of sensors andsubstitution of sensor data between symmetrically positioned instrumenttubes in the core. Both schemes collect data at more core elevationsthan can be achieved by known methods of fabricating GT strings.Reducing the uncertainty in the measurement system favorably impacts theoperating limits for the core and can result in better core performance,lower fuel costs and the elimination of complex TIPs.

The monitoring software uses the asymmetrical arrangement of sensors tosubstitution sensor data between symmetrically positioned instrumenttubes in the core, the monitoring and to calibrate the detectors, e.g.,LPRM detectors in different tubes. This capability allows the softwareto calibrate a greater number of LPRM detectors than the number of fixedGT sensors. The calibration can be conducted frequently to ensure thatthe data generated by the detectors is accurately calibrated. Further,detectors that are not adjacent GT sensors may be calibrated by usingdata from GT sensors that are symmetrically positioned in anotherinstrument tube. While the substitutability of sensor data fromsymmetrical locations in a core is conventional, this disclosurepresents an application of the substitutability of sensor data that waspreviously unknown to the best of the knowledge of the inventors.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A method for collecting data regarding the operating condition of anuclear reactor core, the method comprising: positioning a first lineararray of gamma thermometer (GT) sensors in a first instrument housing,wherein the GT sensors are arranged asymmetrically along the firstlinear array; positioning a second linear array of GT sensors in asecond instrument housing, wherein the GT sensors are arrangedasymmetrically along the second linear array and wherein the secondlinear array of GT sensors is asymmetrical with respect to the firstlinear array of GT sensors; positioning the first instrument housing inthe reactor core at a first core location and positioning the secondinstrument housing at a second core location symmetrical with respect tothe first core location; collecting core condition data from at leastone of the GT sensors in the first linear array of GT sensors, andapplying the collected core condition data as data collected from thesecond linear array.
 2. The method as in claim 1 wherein the applyingthe collected core condition data includes applying the data as havingbeen collected at substantially the same elevation as the at least oneof the GT sensors in the first linear array.
 3. The method as in claim 2wherein the elevation in the second linear array does not have a GTsensor.
 4. The method as in claim 3 wherein the elevation includes adetector for a Local Power Range Monitor (LPRM) adjacent the secondlinear array, and the method includes calibrating the detector using thecollected data from the at least one GT sensor on the first linear arrayat the same elevation.
 5. The method as in claim 1 wherein the secondlinear array includes a majority of the GT sensors in a lower half ofthe array and the first linear array includes a majority of the GTsensors in an upper half of the array, and at least one of said GTsensors in the lower half of the array in the first linear array isadjacent each detector of a first LPRM adjacent the first linear arrayand at least one detector in an upper half of the first LPRM has noadjacent GT sensor on the first linear array, and at least one of saidGT sensors in the upper half of the array in the second linear array isadjacent each detector of a second LPRM adjacent the second linear arrayand at least one detector in a lower half of the second LPRM has noadjacent GT sensor from the second linear array.
 6. The method as inclaim 1 wherein the first core location and second core location are ata common distance from a line of symmetry extending through the core. 7.The method as in claim 1 wherein the application of the collected coredata includes generation of a power shape for the core.
 8. A pair oflinear arrays of gamma thermometer (GT) sensors arranged in a nuclearreactor core, the pair comprising: a first linear array of GT sensors,wherein the GT sensors are arranged asymmetrically along a length of thefirst linear array; a second linear array of GT sensors, wherein the GTsensors are arranged asymmetrically along the second linear array andwherein the second linear array of GT sensors is asymmetrical withrespect to the first linear array of GT sensors, and the first lineararray positioned in the reactor core at a first core location and thesecond instrument housing positioned at a second core locationsymmetrical with respect to the first core location.
 9. The pair as inclaim 8 wherein the second linear array includes a majority of the GTsensors in a lower half of the array and the first linear array includesa majority of the GT sensors in an upper half of the array.
 10. The pairas in claim 8 wherein the first core location and second core locationare at a common distance a line of symmetry extending through an axis ofthe core.
 11. A method to collect and present data from gammathermometer (GT) sensors indicative of a nuclear reactor core, themethod comprising: forming a plurality linear GT arrays of GT sensors,wherein an axial positions of the GT sensors in each array is notpredetermined prior to forming the array; determining the axial positionof each of a plurality of GT sensors arranged in each of the linear GTarrays; storing the axial positions for each GT sensors in a data fileassociated with the linear GT array; loading the data file for each ofthe linear GT arrays into a core monitor software positioning; for eachof the linear GT arrays, the core monitor software determines theelevation in the core of each of the GT sensors based on the axialpositions in the data file; positioning the linear GT arrays in thecore; collect data regarding an operating condition of the core from theGT sensors for each linear GT array, and the core monitoring softwareusing the collected data to generate a presentation of a core conditionat various core elevations.
 12. The method of claim 11 wherein thepresentation is a 3-Dimensional (3D) graph of core power at various corenodal positions corresponding to the linear GT arrays.
 13. The method ofclaim 11 further comprising: positioning each of the linear GT arrays ina separate instrument tube; positioning detectors for a Local PowerRange Monitor (LPRM) in each of the instrument tubes; for eachinstrument tube, identifying a one of the GT sensors of the arrayadjacent each of the detectors, and calibrating each of the detectors byinterpolating signals from GT sensors proximate to each of thedetectors.
 14. The method of claim 11 wherein forming the pluralitylinear GT arrays includes placing the GT sensors in a metallic rod andextruding the rod with the GT sensors in the rod.
 15. The method ofclaim 11 wherein positioning the linear GT arrays includes arranging apair of the GT arrays at core locations at a same distance from a lineof symmetry extending through a core axis.
 16. The method of claim 15further comprising applying GT data collected from one array of the pairof GT arrays as having been collected at the other array of the pair ofGT arrays.
 17. The method of claim 15 wherein at least two of the GTarrays are positioned at a common distance from a line of symmetryextending through an axis of the core.
 18. The method of claim 17wherein the at least two of the GT arrays are symmetrically located inthe core, and the method further comprises using data collected from oneof the at least two of the GT arrays as data collected at the other ofthe at least two of the GT arrays.
 19. The method of claim 11 whereineach of the linear GT array has at least four GT sensors, and at leastone GT sensor is in close proximity to a LPRM.
 20. The method of claim11 wherein the GT sensors in each linear GT array are asymmetricallyarranged along an axis of the array.